I recently acquired a classic Quad 405-2 power amplifier. She sounded fantastic before overhaul, but read on to see how she sounded after!
The Quad 405-2 is revered by many, even 40 years after its release. It’s not hard to hear how the 405-2 does an amazing job of getting out of the way and just playing the music. This is a truly special amplifier, don’t let the understated English looks fool you.
This amp is a Quad 405-2, but the 405 and 405-2 are quite similar. They are masterpieces of clever, understated design and showcase the less-is-more approach of Quad’s legendary engineer, Peter Walker.
I can’t think of a mainstream commercial amplifier with fewer parts than the Quad 405. Sure, there is an IC in the front-end to keep the parts count down, but the driver stage is just two TO-220 devices operating in class-A. The output stage is another two N-channel TO-3 devices per channel. No complimentary pair mismatching here.
The driver and output transistors are thermally coupled to a block of aluminium. They are board-mounted, eliminating wiring. Add a sprinkling of passive components of very high quality and you have the elegant simplicity that is the Quad 405 amplifier module.
Much has been written about the remarkable Quad 405, and I’ve written a fair bit myself. Check out my review of the Quad 405 / 405-2, and another article I wrote about 405 standard servicing. There are a great review and some background info, here.
One of my favourite resources is Keith Snook’s fantastic Quad repository. Keith breaks things down into two large and very detailed 405 pages. The first covers the original 405 and its various iterations, the second covers the 405-2, and its various iterations. Keith’s site is a wealth of information and you’ll also find the service manual along with all revisions and parts lists, very handy indeed.
Overhaul and Improvement
OK, so what did I do to mine? I tend to utilise a similar, less-is-more approach to my work, not senselessly changing parts that are perfectly good. To that end, transistors, resistors and most other parts remain factory. Quad used excellent quality parts in most places and most are perfect, so why change them?
My 405 needed of some TLC though. The seller told me it was ‘restored’ so I was keen to see what that actually meant. Restored means something very different to me obviously, I guess this is why I have so many customers, but in this context, it’s annoying. The amp was also damaged during shipping. I claimed for this through eBay and was refunded the full purchase price.
Apart from that, mine has two different amp modules, one version apart. Quad made something like 10 revisions of the 405/405-2, improving them as they went. This is of no real consequence in a dual mono amp like this, and I’ve since acquired a matching module, but an original must have blown up at some point. Quad had a simple repair policy – order a new module and screw it in place, job done.
My amp contained a few original and dead caps, some old chewed-out screws and other bits and pieces that I decided to sort out. Let’s have a look inside…
Further Quad 405 Mods…
The 405 sounded much better after replacing the old board-mounted capacitors. I initially decided against changing the TL-071 op-amps for a number of reasons. Firstly, I’m against the forum-driven, non-scientific craze of just swapping op-amps because someone said they should sound better. What’s better, and smarter, is to try to understand the reasons behind the choices originally made. Then one can see if there is a better, modern replacement.
For noise performance, low DC offsets and low distortion, the FET input TL-071 is an excellent choice, and still a great op-amp, despite what you might read. The TL-2071, is an excellent modern replacement and I can verify that in this amplifier they make a significant difference.
The remaining important mod was to replace the original filter capacitors with modern, higher-spec parts. STC branded 10,000uF @ 63V parts came factory-fitted. I replaced these with premium Kemet 15,000uF capacitors. The Kemet parts sound better and are rated for longer service life. Before doing so, I examined the increased inrush current to ensure this was within limitations of the diode bridge.
The original STC filter caps measured quite badly. Capacitance was OK, but ESR was quite high for large caps. We should see an ESR of very close to 0 Ohms for large caps, measured using a proper ESR meter. The STC caps measured around 0.5 Ohms, which is too high. The new Kemet parts were as close to zero as you can get, at around 0.01 Ohms, plus they have 50% greater capacitance, so this is a worthwhile improvement.
There are a couple of improvement I’ve subsequently made and incorporate into most customer jobs. The first is to replace a couple of ceramic caps in the critical bridge circuit with polystyrene or silver mica parts. Second is to replace the TL-071 input op-amps with the modern TL-2071 equivalent. Other FET input op-amps work as well. I’ve had great success with Texas Instruments OPA-604.
Finally, the Chassis…
Because of the damage incurred during shipping and also because the original screws just looked old, I decided to replace them with new zinc-plated parts. New fasteners were easy to find at my local fasteners supplier, yours too I’m sure. They made a big difference to the appearance of my Quad 405-2.
The last thing is a new side panel to replace the dented one, ordered from Quad UK and on its way. That isn’t here yet and so does not feature in these images – yet!